1. Technical Field
This invention is related to compact micro-wave satellite antennas and automatic antenna positioning systems for tracking a satellite from a moving vehicle.
2. Background Art
Attitude control systems for mobile antennas in satellite communication systems are disclosed in U.S. Pat. Nos. 5,061,963, 4,873,526 and 4,725,843. In these devices, the antenna includes a feed horn facing a conical reflector dish. In order for the reflector dish to capture an adequate signal from the satellite, it must be rather large, typically on the order of a few hundred wave lengths across, resulting in the ungainly and large mobile antenna systems illustrated in the above-referenced patents. The relatively large reflector size provides an adequate antenna gain, arising from the directionality of the antenna gain pattern. The antenna must be pointed directly at the satellite in order to receive an adequate signal therefrom. Thus, such mobile antenna systems must have an attitude control system which insures that the antenna points directly at the satellite to within only a few degrees error in azimuth and elevation. For a geostationary satellite, one might assume that there would be no change in the elevational angle to which the antenna may be aligned. However, since a moving vehicle may pitch significantly, the attitude control system of the antenna must include not only azimuth angle control but also elevation angle control. Alternatively, if the motion of the vehicle can be restricted to avoid any significant pitching, the elevational angle control may be dispensed with. However, it is not always practical to restrict the vehicle motion. Three-dimensional antenna direction control using complex antenna control systems is disclosed in U.S. Pat. Nos. 4,823,134 and 4,630,056. Such antenna control systems suffer from the disadvantage of being very complex and therefore unwieldy.
The mobile antennas of the type illustrated in the above-referenced patents typically are tuned to have a peak gain at a specific frequency. The design of such antennas and their response becomes very critical at extremely high frequencies such as K-band and Ka-band frequencies (on the order of 20 and 30 GHz, respectively). A severe problem is encountered when it is desired to transmit signals to the satellite at one frequency (for example at Ka-band frequency) and to receive signals from the satellite at another frequency (for example at K-band frequency). The microwave components of the antenna, particularly the feed-horn assembly facing the reflector dish, are typically tuned to a specific transmitting or receiving frequency, and are not suitable for handling two extremely different frequencies (such as frequencies lying in two different bands).
Thus, it has seemed that a mobile satellite-tracking antenna requires a relatively large antenna size (including a reflector dish on the order of a few hundred wavelengths across) and a complex antenna attitude control system to maintain antenna alignment with the satellite in two dimensions, while permitting the ground vehicle on which the antenna is mounted to move through significant pitch and yaw angles. Moreover, it does not seem practical to accommodate two different frequency channels lying in different bands (such as K-band and Ka-band signals) using the same antenna.
Accordingly, one object of the invention is to provide a mobile satellite-tracking antenna system in which the antenna size is greatly reduced from that of the present state of the art with an optimum antenna gain or antenna performance.
It is another object of the invention to provide a mobile satellite-tracking antenna capable of transmitting or receiving signals with respect to an orbiting satellite in two different channels or different communication bands such as the K-band and the Ka-band using the same feed-horn assembly and the same reflector dish and attain similar RF performance for both bands.
It is a further object of the invention to provide a mobile satellite-tracking antenna having a very simple low bandwidth control system for maintaining antenna orientation with respect to an orbiting satellite, particularly a geostationary satellite.
It is a related object of the invention to provide a high performance dual-band mobile satellite-tracking antenna which requires antenna attitude control in azimuth only.
It is a yet further object of the invention to provide a mobile satellite-tracking antenna having less elevational directionality to provide low-loss performance over large pitch angles of the ground vehicle on which the antenna is mounted.
It is a still further object of the invention to provide a mobile satellite-tracking antenna having an antenna attitude control system for maintaining antenna orientation with respect to a geostationary satellite, requiring only a ground vehicle yaw angle sensor and an antenna azimuth angle sensor.
It is a still further object of the invention to provide a satellite-tracking antenna having a feed-horn assembly capable of simultaneously feeding signals in K-band and the Ka-band frequency ranges to a reflecting dish on the order of only several to ten wavelengths in extent while requiring attitude control in azimuth only and requiring only an inertial vehicle yaw angle sensor and antenna azimuth angle sensor while maintaining fine azimuth direction control.
The forgoing objects would fulfil the goal of an extremely light-weight compact mobile antenna system mountable on the roof of a small vehicle for tracking the Advanced Communication Technology Satellite (ACTS) which transmits Ka-band signals to the mobile antenna and receives K-band signals from the mobile antenna.